In electrochemical cells such as lithium batteries, the charge carrying entity of interest is the metallic cation, such as lithium, while the anion is not involved with the current-producing electrochemical processes at either electrode. Any transport of charge by the counterion, (the anion in the case of a lithium battery) is undesirable because it leads to concentration polarization of the electrolyte in the cell reducing the charge/discharge rate capability of the cell. In the ideal lithium cell with high power density, lithium ions would be of high concentration and exhibit high mobility, resulting in high conductivity while the counterion would exhibit virtually no mobility, thereby resulting in a high lithium transference number where lithium transference number refers to the number of moles of Li+ transferred for the passage of 1 faraday of electricity.
Rechargeable lithium-ion cells typically use liquid or gelled polymer electrolytes consisting of inorganic lithium salts, such as LiPF6, in an organic solvent. Such electrolytes exhibit high conductivity but the anions have high mobility, often exhibiting a Li transference number of around 0.3.
Similar trade-offs exist when solutions of organic salts having larger counterions, such as perfluoroalkylsulfonates, sulfonimides, and sulfonyl methides such as are disclosed in Waddell et al, U.S. Pat. No. 5,514,493. With most of these salts the transference number of Li is lower than 0.5.
Another approach is to employ solid polymer electrolytes, where the lithium ion is a labile ion attached to a polymeric chain which acts as the anion. See, for example, Narang et al, U.S. Pat. No. 5,633,098 and DesMarteau, U.S. Pat. No. 5,463,005. These materials typically provide very high lithium transference numbers and the cells would exhibit virtually no concentration polarization under load. However, low inherent ionic conductivities, insufficient electrochemical stability, and difficulty in processing limit their usefulness.
Hamrock et al, U.S. Pat. No. 6,063,522 discloses solutions of hybrid fluorocarbon and hydrocarbon imide and methide salts. Included are polymers formed by free-radical polymerization of monomers having hydrocarbon backbones and pendant groups of fluorinated sulfonyl imides linked thereto by carbonyl or phenylsulfonyl connecting groups. Also included are copolymers formed by copolymerizing said monomers with low polarity olefinic comonomers.
Grafting of various monomers onto hydrocarbons by free radical methods is disclosed in J. Polym. Sci., Part A: Polym. Chem. (1997), 35, 3517, Polym. Sci., Part A: Polym. Chem. (1999), 37, 3817, and Polym. Sci., Part A: Polym. Chem. (2000), 38, 2456. In these studies hydrocarbons were used as models for polyolefins. No mention of fluorocarbon monomers is made.
Cripps, U.S. Pat. No. 5,032,306 discloses free radical grafting of perfluoroalkenes and perfluorovinyl ethers onto hydrocarbons having at least four carbon atoms. Examples include hexafluoropropylene and perfluoropropyl vinyl ether monomers. No mention is made of monomers containing perfluoroalkylsulfonate, sulfonimide, or sulfonyl methide groups.